The Problem with the Linpack Benchmark Matrix Generator

We characterize the matrix sizes for which the Linpack Benchmark matrix generator constructs a matrix with identical columns

Ab-initio Gutzwiller method: first application to Plutonium

Except for small molecules, it is impossible to solve many electrons systems without imposing severe approximations. If the configuration interaction approaches (CI) or Coupled Clusters techniques \cite{FuldeBook} are applicable for molecules, their generalization for solids is difficult. For materials with a kinetic energy greater than the Coulomb interaction, calculations based on the density functional theory (DFT), associated with the local density approximation (LDA) \cite{Hohenberg64, Kohn65} give satisfying qualitative and quantitative results to describe ground state properties. These solids have weakly correlated electrons presenting extended states, like $sp$ materials or covalent solids. The application of this approximation to systems where the wave functions are more localized ($d$ or $f$-states) as transition metals oxides, heavy fermions, rare earths or actinides is more questionable and can even lead to unphysical results : for example, insulating FeO and CoO are predicted to be metalic by the DFT-LDA..

Dynamics of Quantum Noise in a Tunnel Junction under ac Excitation

We report the first measurement of the \emph{dynamical response} of shot noise (measured at frequency $\omega$) of a tunnel junction to an ac excitation at frequency $\omega_0$. The experiment is performed in the quantum regime, $\hbar\omega\sim\hbar\omega_0\gg k_BT$ at very low temperature T=35mK and high frequency $\omega_0/2\pi=6.2$ GHz. We observe that the noise responds in phase with the excitation, but not adiabatically. The results are in very good agreement with a prediction based on a new current-current correlator.Comment: Theory removed. More experimental details. One extra figur

Electronic transport in AlMn(Si) and AlCuFe quasicrystals: Break-down of the semiclassical model

The semi-classical Bloch-Boltzmann theory is at the heart of our understanding of conduction in solids, ranging from metals to semi-conductors. Physical systems that are beyond the range of applicability of this theory are thus of fundamental interest. It appears that in quasicrystals and related complex metallic alloys, a new type of break-down of this theory operates. This phenomenon is related to the specific propagation of electrons. We develop a theory of quantum transport that applies to a normal ballistic law but also to these specific diffusion laws. As we show phenomenological models based on this theory describe correctly the anomalous conductivity in quasicrystals. Ab-initio calculations performed on approximants confirm also the validity of this anomalous quantum diffusion scheme. This provides us with an ab-initio model of transport in approximants such as alpha-AlMnSi and AlCuFe 1/1 cubic approximant.Comment: 11 pages, 5 figure

Improving the modelling of redshift-space distortions - II. A pairwise velocity model covering large and small scales

We develop a model for the redshift-space correlation function, valid for both dark matter particles and halos on scales $>5\,h^{-1}$Mpc. In its simplest formulation, the model requires the knowledge of the first three moments of the line-of-sight pairwise velocity distribution plus two well-defined dimensionless parameters. The model is obtained by extending the Gaussian-Gaussianity prescription for the velocity distribution, developed in a previous paper, to a more general concept allowing for local skewness, which is required to match simulations. We compare the model with the well known Gaussian streaming model and the more recent Edgeworth streaming model. Using N-body simulations as a reference, we show that our model gives a precise description of the redshift-space clustering over a wider range of scales. We do not discuss the theoretical prescription for the evaluation of the velocity moments, leaving this topic to further investigation.Comment: 18 pages, 10 figures, published in MNRA

Constraining Dark Matter-Neutrino Interactions using the CMB and Large-Scale Structure

We present a new study on the elastic scattering cross section of dark matter (DM) and neutrinos using the latest cosmological data from Planck and large-scale structure experiments. We find that the strongest constraints are set by the Lyman-alpha forest, giving sigma_{DM-neutrino} < 10^{-33} (m_DM/GeV) cm^2 if the cross section is constant and a present-day value of sigma_{DM-neutrino} < 10^{-45} (m_DM/GeV) cm^2 if it scales as the temperature squared. These are the most robust limits on DM-neutrino interactions to date, demonstrating that one can use the distribution of matter in the Universe to probe dark ("invisible") interactions. Additionally, we show that scenarios involving thermal MeV DM and a constant elastic scattering cross section naturally predict (i) a cut-off in the matter power spectrum at the Lyman-alpha scale, (ii) N_eff ~ 3.5 +/- 0.4, (iii) H_0 ~ 71 +/- 3 km/s/Mpc and (iv) the possible generation of neutrino masses.Comment: 12 pages, 5 figure

Optimization of quantum Monte Carlo wave functions by energy minimization

We study three wave function optimization methods based on energy minimization in a variational Monte Carlo framework: the Newton, linear and perturbative methods. In the Newton method, the parameter variations are calculated from the energy gradient and Hessian, using a reduced variance statistical estimator for the latter. In the linear method, the parameter variations are found by diagonalizing a non-symmetric estimator of the Hamiltonian matrix in the space spanned by the wave function and its derivatives with respect to the parameters, making use of a strong zero-variance principle. In the less computationally expensive perturbative method, the parameter variations are calculated by approximately solving the generalized eigenvalue equation of the linear method by a nonorthogonal perturbation theory. These general methods are illustrated here by the optimization of wave functions consisting of a Jastrow factor multiplied by an expansion in configuration state functions (CSFs) for the C$_2$ molecule, including both valence and core electrons in the calculation. The Newton and linear methods are very efficient for the optimization of the Jastrow, CSF and orbital parameters. The perturbative method is a good alternative for the optimization of just the CSF and orbital parameters. Although the optimization is performed at the variational Monte Carlo level, we observe for the C$_2$ molecule studied here, and for other systems we have studied, that as more parameters in the trial wave functions are optimized, the diffusion Monte Carlo total energy improves monotonically, implying that the nodal hypersurface also improves monotonically.Comment: 18 pages, 8 figures, final versio

Zero-variance zero-bias quantum Monte Carlo estimators of the spherically and system-averaged pair density

We construct improved quantum Monte Carlo estimators for the spherically- and system-averaged electron pair density (i.e. the probability density of finding two electrons separated by a relative distance u), also known as the spherically-averaged electron position intracule density I(u), using the general zero-variance zero-bias principle for observables, introduced by Assaraf and Caffarel. The calculation of I(u) is made vastly more efficient by replacing the average of the local delta-function operator by the average of a smooth non-local operator that has several orders of magnitude smaller variance. These new estimators also reduce the systematic error (or bias) of the intracule density due to the approximate trial wave function. Used in combination with the optimization of an increasing number of parameters in trial Jastrow-Slater wave functions, they allow one to obtain well converged correlated intracule densities for atoms and molecules. These ideas can be applied to calculating any pair-correlation function in classical or quantum Monte Carlo calculations.Comment: 13 pages, 9 figures, published versio